Examination of force discrimination in human upper limb amputees with reinnervated limb sensation following peripheral nerve transfer

Abstract

Artificial limbs allow amputees to manipulate objects, but the loss of a limb severs the sensory link between a subject and objects they touch. A novel surgical technique we term targeted reinnervation (TR) allows severed cutaneous nerves to reinnervate skin on a different portion of the body. This technique provides a physiologically appropriate portal to the sensory pathways of the missing limb through the reinnervated skin. This study quantified the ability of three amputee subjects who had undergone TR surgery on the chest (two subjects) and upper arm (one subject) to discriminate changes in graded force on their reinnervated skin over a range of 1-4 N using a stochastic staircase approach. These values were compared to those from sites on their intact contralateral skin and index fingers, and from the chests and index fingers of a control population (n = 10) . Weber's ratio (WR) was used to examine the subjects' abilities to discriminate between a baseline force and subsequent forces of different magnitudes. WRs of 0.22, 0.25, and 0.12 were measured on the reinnervated skin of the three TR subjects, whereas WRs of 0.25, 0.23, and 0.12 were measured on their contralateral skin. TR subjects did not have substantially different WRs on their reinnervated versus their contralateral normal side and did not appear to exhibit a trend towards impaired sensation. No significant difference was found between the WR of the chest and index finger of the control subjects, which ranged between 0.09 and 0.21. WR of reinnervated skin for TR subjects were within the 95% confidence interval of the control group. These data suggest that subjects with targeted reinnervation have unimpaired ability to discriminate gradations in force.

Fig. 1

Force trace of a single trial. The order of the standard stimulus and the modified stimulus was randomized. Subjects were required to respond by saying that either the first or the second stimulus was larger (two alternatives, with a required decision).

Fig. 2

Subject SD1 was tested on two locations: his left reinnervated chest and right normal chest. (a) Force applied to the testing site elicited referred sensations that were projected to the thenar aspect on the dorsal aspect of his missing hand. The right normal chest location was chosen in a place devoid of scar tissue. Figure adapted from [1]. (b) TR subject plots are presented as a function of baseline force for the chest.

Fig. 3

Subject SD2 was tested on three locations: her left reinnervated chest, right chest, and right index finger. (a) Force applied to the testing site elicited referred sensations that were projected to three distinct locations on the palmar aspect of her missing hand. The right chest location was chosen to provide symmetry with the reinnervated location. Figure adapted from [1]. (b) TR subject plots are presented as a function of baseline force for the chest and index finger.

Fig. 4

Subject TH3 was tested on three locations: her left reinnervated arm, right arm, and right index finger. (a) Force applied to the testing site elicited referred sensations that were projected to a broad region of the palmar aspect of her missing hand. The right arm location was chosen to provide symmetry with the reinnervated location. (b) TR subject plots are presented as a function of baseline force for the chest and index finger.